Closed-loop Sagnac phase shift detection approaches for high precision, low drift (approximately 0.01 degrees per hour) and wide dynamic range sensing applications, such as inertial navigational systems, have been developed in recent years. These systems are known to be highly accurate and expensive to construct. Examples are included in Proceedings of the 10th Anniversary Conference on Fiber Gyros, H. Arditty et al., The Society of Photo-Optical Instrumentation Engineers, Vol. 719, Cambridge, Mass., 1986 and in "Fiber Optic Gyroscope with All-Digital Closed-Loop Processing," Proceedings of the Sixth International Conference on Optical Fiber Sensors, OFS '89, pp. 131 et seq., Springer-Verlag, Paris, 1989.
These approaches typically involve complex and high speed electronic signal processing schemes and integrated optical components to provide a precise non-reciprocal phase shift in a highly linear fashion for introduction into the Sagnac interferometer to counterbalance the rotation-induced Sagnac phase shift. Such apparatus has been too complex and prohibitively expensive for less-demanding applications, where minimum detectable rotations of approximately 1 to 10 degrees per hour and linearity of approximately 0.1 to 1 percent may well be adequate.
Open-loop gyroscopes provide a lower cost option for many medium performance applications, but these gyroscopes are limited by a number of factors. For example, the output of an open-loop gyroscope depends sinusoidally on the Sagnac phase shift, and thus the rotation rate. This leads to non-linearity, and also to a limited dynamic range. See "Fiber Optic Rotation Sensors and Related Technologies," Springer Series in Optical Sciences, Vol. 32, S. Ezekiel and J. J. Arditty, editors, New York; Springer-Verlag, 1982 and in "Open-Loop Output and Scale Factor Stability in a Fiber Optic Gyroscope," R. P. Moeller et al , IEEE Journal of Lightwave Technology, Vol. 7, pp. 262 et seq., 1989.
Furthermore, the scale factor depends directly on the source intensity and on the fringe visibility. The output must be normalized to accommodate these factors in order to eliminate bias and scale factor drift. These problems are discussed at greater length in "Pseudo-Hetrodyne Detection Scheme for the Fiber Gyroscope," A. D. Kersey et al., Electronic Letters, Vol. 20, pp. 368 et seq., 1984 and also in "An Amplitude Switched Fiber Optic Gyroscope," N. Frigo, Proceedings of the Fourth International Conference on Optical Fiber Sensors, OFS '86, pp. 181 et seq., OITDA, Tokyo, 1986.
The magnitude of the Sagnac phase shifts is given by Equation 1: ##EQU1## wherein: A is the cross-sectional area of the fiber,
N is the number of fiber turns, PA1 .lambda..sub.o is the wavelength of the light, and PA1 .OMEGA. is the rotation rate.
Generally, higher sensitivity measurements require the detection of phase shifts smaller than 10.sup.-6 radians. Due to the interferometer transfer function, however, the output becomes nonlinear at higher rotation rates, and if .DELTA..phi..sub.s exceeds one-half pi radians the output becomes ambiguous.
In view of the known limitations and shortcomings of the prior art devices, as well as other disadvantages not specifically mentioned above, it should be apparent that there still exists a need in the art for a low-cost electronic phase-tracker for open-loop fiber optic gyroscope apparatus.
It is, therefore, a primary object of this invention to fulfill that need by providing a low-cost, wide dynamic range demodulation technique which is relatively easy and inexpensive to construct.